diff --git a/c/jsonfmt.c b/c/jsonfmt.c
index 051a2e5..c4c8594 100644
--- a/c/jsonfmt.c
+++ b/c/jsonfmt.c
@@ -12,7 +12,7 @@ static void fujson_fmt_str(pTHX_ fustr *out, const char *stri, size_t len, int u
      * strings, I suspect there's room for optimizations in
      * is_c9strict_utf8_string().  */
     if (utf8 && !is_c9strict_utf8_string(str, len)) {
-        return; /* TODO: Throw error. */
+        croak("invalid codepoint encountered in string, cannot format to JSON");
     }
 
     fustr_write(out, "\"", 1);
@@ -151,9 +151,8 @@ static void fujson_fmt(pTHX_ fustr *out, SV *val) {
     } else if (SvPOKp(val)) {
         fujson_fmt_str(aTHX_ out, SvPVX(val), SvCUR(val), SvUTF8(val));
     } else if (SvNOKp(val)) { /* Must check before IOKp, because integer conversion might have been lossy */
-        /* TODO: quadmath? */
         NV nv = SvNV_nomg(val);
-        if (isinfnan(nv)) return; /* TODO: error */
+        if (isinfnan(nv)) croak("unable to format floating point NaN or Inf as JSON");
         fustr_reserve(out, NV_DIG+1);
         Gconvert(nv, NV_DIG, 0, out->cur);
         out->cur += strlen(out->cur);
@@ -165,11 +164,11 @@ static void fujson_fmt(pTHX_ fustr *out, SV *val) {
         if (UNLIKELY(SvOBJECT(rv))) { /* TODO: Check for TO_JSON */ }
         else if (SvTYPE(rv) == SVt_PVHV) fujson_fmt_hv(aTHX_ out, (HV *)rv);
         else if (SvTYPE(rv) == SVt_PVAV) fujson_fmt_av(aTHX_ out, (AV *)rv);
-        else return; /* TODO: error */
+        else croak("unable to format reference '%s' as JSON", SvPV_nolen(val));
     } else if (!SvOK(val)) {
         fustr_write(out, "null", 4);
     } else {
-        /* TODO: error */
+        croak("unable to format unknown value '%s' as JSON", SvPV_nolen(val));
     }
 }
 
diff --git a/t/json_format.t b/t/json_format.t
index b5334a2..8493dec 100644
--- a/t/json_format.t
+++ b/t/json_format.t
@@ -50,7 +50,15 @@ my @tests = (
     1844674407370955161 / 10, '1.84467440737096e+17',
 );
 
-plan tests => @tests + 6;
+my @errors = (
+    \1, qr/unable to format reference/,
+    *STDOUT, qr/unable to format unknown value/,
+    'NaN'+0, qr/unable to format floating point NaN or Inf as JSON/,
+    'Inf'+0, qr/unable to format floating point NaN or Inf as JSON/,
+    do { no warnings 'portable'; "\x{ffffffff}" }, qr/invalid codepoint encountered in string/,
+);
+
+plan tests => @tests + @errors/2 + 6;
 
 for my($in, $exp) (@tests) {
     my $out = json_format $in;
@@ -58,6 +66,11 @@ for my($in, $exp) (@tests) {
     ok utf8::is_utf8($out);
 }
 
+for my ($in, $exp) (@errors) {
+    eval { json_format $in };
+    like $@, $exp;
+}
+
 
 
 # http://e-choroba.eu/18-yapc slide 6